Answer

The 'locking timed-out' message means that the mailbox in question has been opened for writing by a thread and another thread wants to write to it as well. The second thread tries to acquire the lock, but times out. This means that the first operation is still busy - for more than 30 seconds. Usually this means either it's an extremely large mailbox (without indexing), or there is some problem with the file system (very slow response).

Please make sure that mailbox indexing is enabled (WebAdmin -> Settings -> Other -> TextMailbox manage -> Index -- set to 3 MBs or lower) and that accounts that are set to forward mail are also set to discard already forwarded mail ("Keep Copy" is Off) so mailboxes don't grow beyond a practical limit (of say 1 to 2 GB).